Electrochemical preparation of boron



United States Patent 2,848,396 ELECTROCHEMICAL PREPARATION OF BORON NoDrawing. Application February 4, 1955 Serial No. 486,274

Claims. (Cl. 204-60) This invention relates to the preparation ofelemental boron and more particularly to its preparation by theelectrolysis of a fused salt mixture containing alkali metal oxides andboron compounds.

Several methods for preparing boron have been developed by variousinvestigators in the past. Moissan first prepared boron in 1892 by thereduction of boric oxide with magnesium. Goss developed a method forpreparing boron from boron trichloride and hydrogen. Both these methodsare objectionable either because of the low yields obtained or thedifliculty of separating the boron from other by-products formed. Cooperin United States Patents 2,572,248 and 2,572,249 produced boron of highpurity by the electrolysis of a mixture of potassium salts with orwithout boric oxide at temperatures of 650- 1000 C. However, he claimedthat the use of potassium salts was absolutely essential and that ifsodium salts were substituted in whole or in part the process wascommercially impractical if not tot-ally inoperative. We have found,however, that when alkali metal oxides are used that either sodium orpotassium salts can be used with equal effectiveness.

It is an object of this invention to provide a new and improved methodfor preparing elemental boron by the electrolysis of a fused saltmixture containing boric oxide, alkali metal fiuo'borates and alkalimetal oxides.

Other objects will become apparent throughout the specification andclaims hereinafter related.

This new and improved method will be more fully described in thespecification and the novelty thereof will be particularly pointed outand distinctly claimed.

This invention is based upon our discovery that a mixture of boric oxideand an alkali metal fluoborate containing a substantial amount of analkali metal oxide will produce good yields of elemental boron whenelectrolyzed at a temperature of 750-875 C. Since the alkali metalcarbonates decompose to the oxide in the presence of B 0 at thistemperature, the process is economically attractive.

In carrying out this process, the general procedure was as follows: thesalts to be used were weighed, mixed and .melted together at about 900C. in a graphite crucible which was 4" high and had an outside diameterof 4". A calibrated thermocouple was inserted in a hole drilled in thewall of the crucible. The temperature was then lowered to that desiredfor the experiment. A low-carbon iron rod was used as the cathode whichwas lowered into the crucible which Was used :as the anode. The directcurrent was regulated and electrolysis allowed to proceed for one houror more With occasional adjustment of heat and current to maintain thedesired experimental conditions. Heat was supplied by a gas cruciblefurnace. At the end of the electrolysis, the current was shut off, thecathode quickly withdrawn and covered with dry sodium chloride toprotect the hot product from oxidation. When cool, the cathode wasimmersed in water until most of the product fell ofi and the remainderwas readily scraped olf. The product was digested in hot water for fourhours, then 2,848,395 Patented Aug. 19, 1958 2. refluxed in stronghydrochloric acid, washedagain with water, dried and weighted. Theweight of the residue was recorded as crude product. The cathodedeposits obtained were analyzed for boron as Well as carbon and otherimpurities.

In one series of experiments, a mixture of 2 mols of K 00 2 mols of KBFand 1 mol of B 0 was melted in a graphite crucible at 900950 C. untilall of the CO was evolved. The temperature was then decreased to 850 C.and a direct current of 15 amperes was passed from the graphite crucibleanode to the steel cathode of 4 sq. in. immersed area. After two hours,the steel cathode was removed and cooled under a layer of sodiumchloride. The cathode was soaked in water and the deposit removed byscraping it off. The deposit was digested in hydrochloric acid,filtered, washed and dried. The analysis of the product in threeseparate runs was as follows:

Percent Wt. in Oxygen Run N0. grams Percent Percent and Other CrudeCarbon Boron Impurities Product by Difference In another series ofexperiments using a 1:1:1 molar ratio of K CO to KBF to B 0 andelectrolyzing the mixture at 850-875 C. using a direct current of 20amperes for 1.66 hours the following results were obtained:

Percent Wt. in Oxygen Run No. grams Percent Percent and Other CrudeCarbon Boron Impurities Product by Difference In still another series ofruns using Na CO in place of K CO and NaBF, in place of KBF in a ratioof 1 mol of Na CO to 1 mol of NaBF to 1 mol of B 0 and electrolyzing themixture at various temperatures with a direct current of 15-20 amperesfor various lengths of time the following results were obtained:

Percent Wt. in Oxygen Run 'Iemp., Time, grams Percent Percent and OtherNo. C. Hrs. Crude Carbon Boron Impurities Product by Difference Itshould be noticed that a low yield and a poor quality product wasobtained at a temperature below 800 C. since the bath becomes tooviscous for efficient electrolysis. When optimum conditions are used theonly appreciable impurity that exists is carbon which results fromdecrepi- 3 at 800 -820 C. The products in these runs all contained ahigh percentage of boron.

Oxygen is the anode product according to the equation 3M O 6M++1 /2O +6eWhen a graphite anode is used carbon monoxide is the main anode productfrom reaction of the anode with the electrochemically produced oxygenaccording to 3C+ /zO 3CO.

It should be pointed out that the main advantages of this process are(1) no chlorine is evolved at the anode, (2) potassium salts are notessential, (3) less Water is absorbed by the sodium salt mixtures onstanding than when potassium salts are used, (4) sodium salt mixturesdecrease corrosion of the metal cathode, and (5) the use of sodium saltmixtures reduces erosion of the anode and the crucible.

Having thus described our invention fully and completely as required bythe patent statutes, what we desire to claim and secure by United StatesPatent is:

1, A method of preparing elemental boron which comprises heating amixture of K CO KBE; and B 0 in a molar ratio of 1:1:1 to 2:2:1 at atemperature in excess of 900 C. until all the CO is evolved, thenelectrolyzing the resulting fused mixture at a temperature of 850- 875C. using a graphite anode and a steel cathode and recovering the borondeposited at the cathode.

2. A method of preparing elemental boron which comprises heating amixture of Na CO NaBF and B 0 in a molar ratio of 1:1:1 at a temperaturein excess of 900 C. untill all the CO is evolved, then electrolyzing theresulting fused mixture at a temperature of 750-875 C. using a graphiteanode and a steel cathode and recovering the boron deposited at thecathode.

3. A method of preparing elemental boron which comprises heating amixture of K CO NaBF and B 0 in a molar ratio of 1:1:1 at a temperaturein excess of 900 C. until all the CO is evolved, then electrolyzing theresulting fused mixture at a temperature of about 850 C. using agraphite anode and a steel cathode and recovering the boron deposited atthe cathode.

4. A method of preparing elemental boron which comprises heating amixture of Na CO KBE; and B 0 in a molar ratio of 1:1:1 at a temperaturein excess of 900 C. until all the CO is evolved, then electrolyzing theresulting fused mixture at a temperature of 800-820 C. using a graphiteanode and a steel cathode and recovering the boron deposited at thecathode.

5. A method of preparing elemental boron which comprises electrolyzing afused mixture consisting essentially of boric oxide, alkali metalfluoborate and alkali metal oxide, the molar ratio of alkali metal oxideto alkali metal fluoborate to boric oxide being in the range of 1:1:1 to2:2: 1, at a temperature in excess of 750 C. and recovering the borondeposited at the cathode.

References Cited in the file of this patent UNITED STATES PATENTS2,572,249 Cooper Oct. 23, 1951 FOREIGN PATENTS 37,559 France Sept. 23,1930 OTHER REFERENCES Gore: Chemical Society Journal (London), vol. 48(1885 Abstracts), pages and 111,

5. A METHOD OF PREPARING ELEMENTAL BORON WHICH COMPRISES ELECTROLYZING AFUSED MIXTURE CONSISTING ESSENTIALLY OF BORIC OXIDE, ALKALI METALFLUOBORATE AND ALKALI METAL OXIDE, THE MOLAR RATIO OF ALKALI METAL OXIDETO ALKALI METAL FLUOBORATE TO BORIC OXIDE BEING IN THE RANGE OF 1:1:1 TO2:2:1, AT A TEMPERATURE IN EXCESS OF 750*C. AND RECOVERING THE BORONDEPOSITED AT THE CATHODE.